Switchable roller finger followers have an outer lever pivotably mounted outside an inner lever and a roller rotatably mounted on a transverse axle in a slot in the inner lever. An example of a switchable finger follower is described in U.S. Pat. No. 7,174,869. In this case, the top surface of the outer lever can act as a contact surface for a high lift cam and the top surface of the roller acts as a contact surface for a low lift cam. A coupling element is mounted at one end of the finger follower. When the coupling element is activated, it locks the outer lever to the inner lever and requires the follower to follow the high lift cam and transfer the lift to the valve stem of an associated intake or exhaust valve. When the coupling element is deactivated, the outer lever is free to pivot relative to the inner lever, with the motion being absorbed by a lost-motion spring, and the motion of the low lift cam is transferred by the inner lever to the valve stem. In other known arrangements used for cylinder deactivation, the coupling element maintains the inner and outer levers connected in order to transfer lift from the cam to the valve stem for the associated intake or exhaust valve when a cylinder is active, and the coupling element is released by oil pressure for disengaging the inner lever from the outer lever so that the inner lever travels a lost motion stroke when the cylinder is deactivated so that the associated intake or exhaust valve remains closed.
The coupling element is conventionally activated and deactivated by hydraulic pressure. For example, the switchable finger followers may be activated or deactivated by pressurized hydraulic fluid that is fed through a feed path from a switching oil gallery, through a hydraulic lash adjuster, and to an actuator chamber in the roller finger follower. The rise in hydraulic pressure in the actuator chamber pushes the coupling element to the activated or deactivated position. When the hydraulic pressure is reduced, a biasing element returns the coupling element to the respective deactivated or activated position.
One known problem with using hydraulic pressure in a switchable roller finger follower is that there can be a lag time for actuation of the switching function due to air bubbles in the hydraulic fluid in the switching oil gallery or the switching oil hydraulic fluid path to the coupling element actuator. These air bubbles delay the switching time, which is unsuitable for devices which require a short switching time. This also leads to a lack of consistency and repeatability in the switching time.
The present disclosure is directed to overcoming these and other problems of the prior art, including those associated with air being present in the hydraulic flow path of a switchable roller finger follower.